TheBitPriest wrote a wonderful walkable areas and pathfinding article on the wikipages, explaining what to do to ensure your walkable area is drawn correctly, and you won't experience any pathfinding issues. Copied a part of the wiki article for your eyes below.
Knowing a few details about the inner workings of AGS's path-finding algorithm can be helpful when designing walkable areas. Some designers have been frustrated by what would seem to be random reasons for a player-character not to be able to navigate to a desired location. For instance, a player-character may be able to successfully navigate a path from one direction and not the other, resulting in the character getting stuck in portions of a room.
The Easiest and Optimal Solution
The easiest way to ensure that you have an optimized walkable area is to make certain that your area fits within the following 3x3 grid. You can expand the grid by pasting together more points at the same aspect ratio.